Multilayer acoustic impedance converter for ultrasonic transducers

ABSTRACT

An impedance conversion layer useful for medical imaging ultrasonic transducers comprises a low impedance polymer layer and a high impedance metal layer. These layers are combined with corresponding thicknesses adapted to provide a function of converting from a specific high impedance to specific low impedance, wherein the polymer layer is at the high impedance side and the metal layer is at the low impedance side. The effective acoustic impedance of the polymer and metal layer combination may be adapted to configure an impedance converter in the same way as a quarter wavelength impedance converter, converting from low impedance to high impedance (metal to polymer) or from a high impedance to low impedance (polymer to metal). This structure may be used for front matching with the propagation medium and back matching with an absorber for ultrasonic transducers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/238,816, filed Sep. 1, 2009, which applicationis incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to acoustic impedance convertersfor an ultrasonic transducer and methods for designing the same.

BACKGROUND OF THE INVENTION

Ultrasonic transducers are often used as impulse mode transducersoperating over a wide range of frequencies. Since such transducers needto handle wideband frequency signals, wideband design is an importantsubject. In the prior art, impedance converters have been placed on aface of a piezoelectric layer of an ultrasonic transducer to improve thewideband frequency response of the transducer. One of the importantapplications of wideband transducers is in medical imaging systems.Economical, reliable and reproducible mass-production processes fortransducers for use in medical imaging systems are particularlydesirable.

Impedance converters for ultrasonic transducers are known in the art. Asis known in the art, an ultrasonic transducer includes a piezoelectricactive layer, one or more front matching layers on a front face of thepiezoelectric active layer to serve as an impedance converter, and abacking absorber on a rear face of the piezoelectric active layer. Atypical piezoelectric material, such as lead zirconate titanate (alsoknown as “PZT”) has high characteristic acoustic impedance, for example,Z_(PZT)=30×10⁶ kg/m²s (Rayl). A typical propagation medium, such aswater, has low characteristic acoustic impedance, for example,Z_(R)=1.5×10⁶ Rayl. Because of the difference in characteristic acousticimpedances of these media, acoustic waves in the piezoelectric activelayer of an ultrasonic transducer are reflected backward into thepiezoelectric active layer at the boundary between the piezoelectricactive layer and the transmission medium (the front boundary) andreflected frontward into the piezoelectric active layer at the backboundary (the boundary between the rear face of the piezoelectric activelayer and the material to the rear of the piezoelectric active layer).This results in a resonance at a specific frequency in the ultrasonictransducer, as determined by the half wavelength condition of thepiezoelectric material.

When such a resonated transducer is driven by a voltage pulse (whenacting as a transmitter) or by an acoustic pulse (when acting as areceiver), the signal wave does not decay quickly (a phenomenon known asringing). This effectively renders such a transducer unsuitable forimaging systems, in which systems short acoustic pulse beams areexcited, directionally scanned and reflected back from a target toenable an image of the target to be constructed. A front impedanceconversion layer (also known in the art as a matching layer for reducingreflections) is inserted between the front face of the piezoelectriclayer and the propagation medium to mitigate creation of resonance dueto the difference in the characteristic acoustic impedances of thepiezoelectric material and the front propagation medium.

A piezoelectric layer's vibration excites an acoustic wave in thebackward direction, i.e., in a direction away from the front face of thepiezoelectric layer. A certain amount of reflection from the backboundary towards the front face may be desirable to improve thesensitivity of the ultrasonic transducer. Often a backing absorber layerof acoustic absorber material is attached to the rear face of thepiezoelectric layer. If the characteristic acoustic impedance of thebacking absorber material effectively matches that of the piezoelectricmaterial, a significant amount of acoustic wave energy passes throughthe back boundary without reflection and is absorbed by the backingabsorber layer. In such a case, the sensitivity of the transducer islowered and the bandwidth may become excessive for some applications.Therefore, some mismatch between the characteristic acoustic impedanceof the piezoelectric material and the backing absorber material isdesirable, depending on the required bandwidth and sensitivity.

The characteristic acoustic impedance of the backing absorber materialmay be selected to obtain a desired performance of the ultrasonictransducer. If a transducer cannot be provided with a backing absorbermaterial of a suitable characteristic acoustic impedance, a backimpedance conversion layer may be added between the piezoelectric activelayer and the backing absorber layer to provide the necessary overallacoustic impedance at the back boundary of the piezoelectric layer.

A typical acoustic impedance conversion structure may be a layer ofuniform thickness, the thickness equal to about one-quarter of thewavelength of a desired operating wavelength of the acoustic transducer.The acoustic impedance conversion layer has a characteristic acousticimpedance (Z_(m)), which is approximately the geometric mean of thecharacteristic acoustic impedance (Z₁) of the propagation medium and thecharacteristic acoustic impedance (Z_(p)) of the piezoelectric activelayer, i.e., Z_(m)=√(Z₁·Z_(p)). Since Z₁ is small (Z₁=Z_(R)=1.5×10⁶Rayl), and the characteristic acoustic impedance of the piezoelectriclayer is relatively high, the characteristic acoustic impedance Z_(m) ofthe matching layer is selected to be between those of the propagationmedium and the piezoelectric layer, i.e., Z_(p)>Z_(m)>Z₁.

One problem associated with a conventional ultrasonic acoustic impedanceconversion layer (i.e., quarter wavelength layer) is the difficulty inchoosing a material to obtain an appropriate characteristic acousticimpedance Z_(m) for both front and back acoustic impedance conversionlayers. More specifically, ultrasonic transducers are often required tooperate over a wide bandwidth (for example, 40-60% of the centerfrequency). To obtain satisfactory performance over such a widebandwidth using bulk PZT as the piezoelectric active layer, a typicalacoustic impedance conversion layer structure used comprises a singlefront matching layer having a characteristic acoustic impedance ofZ_(m)=6.7×10⁶ kg/m²s (Rayl).

Another known acoustic impedance conversion structure providing stillwider bandwidth uses double matching layers. Here, two quarterwavelength layers having characteristic acoustic impedance of Z_(m1) andZ_(m2) are used. In a structure employing double matching layers, thematching layer with characteristic acoustic impedance Z_(m1) is incontact with the propagation medium, which has a characteristic acousticimpedance Z₁; the matching layer with characteristic acoustic impedanceZ_(m2) contacts the surface of the piezoelectric layer. The materials ofthe matching layers are chosen to satisfy a specific relation such asZ_(p)>Z_(m2)>Z_(m1)>Z₁. However, it is quite difficult to obtainappropriate materials for these layers while satisfying the specificdesigned values of the characteristic acoustic impedances. For example,polyimide has a characteristic acoustic impedance of 3.16×10⁶ Rayl.Polyester has a characteristic acoustic impedance of 3.4×10⁶ Rayl, PVDF:3.7×10⁶ Rayl, glass: 13.2×10⁶ Rayl, and aluminum: 17.3×10⁶ Rayl. Inaddition to choosing a material for the front matching layer having asuitable characteristic acoustic impedance, the material shoulddesirably meet other criteria such as process compatibility, ease ofmass-production, and material cost. In the prior art, epoxy loaded withhigh characteristic acoustic impedance material such as glass fiber orsilica powder has been used. However, the thickness and uniformity ofsuch a loaded epoxy proves difficult to control.

Another problem associated with the conventional design of ultrasonictransducers arises in array transducers, where a flexible printedcircuit layer or board on which multiple conductor traces are formed isdisposed to the rear of the array. Each conductor trace is connected toone element of the array. A backing absorber is then attached on therear face of the flexible printed circuit board. The acousticperformance of the flexible printed circuit negatively affects theperformance of the transducer. The polymer layer of a typical flexibleprinted circuit board has characteristic acoustic impedance of about3.2×10⁶ Rayl, which is too low and renders the structure insufficient toserve as an adequate matching layer.

When a piezoelectric layer is diced to define an array of elongatedelements of narrow width, the kerfs or channels between the elements arefilled by a filler material (such as epoxy). As a result, thecharacteristic acoustic impedance of the piezoelectric layer is reduced.In ultrasonic transducers employing such arrays, the properties ofsuitable acoustic impedance converters are different from the propertiesof acoustic impedance converters suitable for transducers having solidpiezoelectric active layers. The selection of suitable materials for theacoustic impedance converters is also dependent on bandwidth andsensitivity requirements. Adjusting the characteristic acousticimpedance Z_(m) of acoustic impedance converters using availabletechniques has proven difficult.

In ultrasonic transducers with no backing absorber, or with air or avery low characteristic acoustic impedance material as a backingabsorber, strong reflections from the back boundary causes thetransducer to operate with a relatively narrow resonance, or results ina strong resonance peak. In such ultrasonic transducers, the fabricationof an appropriate acoustic impedance converter for the front face mayrequire high quality workmanship and custom materials. When an acousticimpedance converter for the front face of the piezoelectric layer isproperly designed and fabricated, a broadband and high efficiencytransducer can be produced. However, large scale production of suchtransducers is difficult to attain due at least in part to the need forskilled artisans having high quality workmanship and custom materials tocreate such acoustic impedance converters.

The concept of a multilayer acoustic impedance converter having a lowcharacteristic acoustic impedance layer arranged closer to apiezoelectric layer and a high characteristic acoustic impedance layerbonded at the outer surface of the low acoustic impedance layer is alsoknown in the art. In the prior art, both layers are less than onequarter of a wavelength thick. The combined structure provides aneffective acoustic impedance conversion equivalent to that of a quarterwavelength scheme. U.S. Pat. No. 6,772,490 teaches multilayer acousticimpedance conversion layers with such a combination of lower and highercharacteristic acoustic impedance layers. The effective characteristicacoustic impedance of the multilayer impedance converter of the '490patent is lower than the characteristic acoustic impedance of theradiation or propagation medium for achieving high sensitivity whenoperating the transducer at the center resonant frequency. While thisdesign is suitable for effective energy transfer at the center frequencyof a narrow bandwidth (which is often suitable for continuous waveexcitation) this design exhibits a steep drop in performance as thefrequency is changed away from the center frequency. Such design isunsuitable for operating the transducer at broader bandwidths requiredfor applications such as pulse excitation and reception.

Another example of a prior art transducer arrangement is provided inToda, “New Type of Matching Layer for Air-Coupled UltrasonicTransducers,” IEEE Transactions on Ultrasonics, Ferroelectrics andFrequency Control, vol. 49, no. 7, July 2002, pp 972-979, whichdescribes a basic design principle of a multilayer acoustic impedanceconverter for reducing reflection at the front of a piezoelectric layerof a transducer with wideband performance. This is an air acoustic wavetransducer. Here, a lower characteristic acoustic impedance layer(formed of air) is disposed at a first surface of the piezoelectriclayer and is followed by a higher characteristic acoustic impedancelayer (formed of a polymer) contacting the propagation medium of air.Each of these layers is thinner than one quarter wavelength and thecombination of these two layers functions as a quarter wavelengthimpedance converter. For an ultrasonic transducer with water or thehuman body as the propagation medium having broad bandwidth operation asrequired for pulse excitation and reception, alternative materials andmethods of implementing such transducers are desired.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, an ultrasonictransducer comprises a piezoelectric element having a characteristicacoustic impedance and a front acoustic impedance converter coupled tothe piezoelectric element. The front acoustic impedance convertercomprises a front polymer layer having a thickness t_(p1) coupled to thepiezoelectric element; and a front metal layer for transmitting acousticenergy between the front polymer layer and a propagation medium having acharacteristic acoustic impedance. The front metal layer has a thicknesst_(m1) and is coupled to the front polymer layer. The characteristicacoustic impedance of the propagation medium is lower than thecharacteristic acoustic impedance of the piezoelectric element, and thefront acoustic impedance converter has an effective characteristicacoustic impedance Z_(C) between the piezoelectric element and thepropagation medium characteristic acoustic impedances. Transmitting ofacoustic energy between the front polymer layer and the propagationmedium may take the form of the ultrasonic transducer operating as atransmitter, a receiver, or a transceiver.

In an embodiment of the invention, the thicknesses of the polymer layerand the metal layer are selected so as to provide the impedanceconverter with the effective characteristic acoustic impedance Z_(C1)based on the densities of the front metal and front polymer layer, theeffective characteristic acoustic impedance Z_(C1), a center resonantfrequency of the ultrasonic transducer and the velocity of sound in thefront polymer layer. According to another embodiment of the invention,the ultrasonic transducer further includes a backing absorber coupled tothe piezoelectric element, wherein the backing absorber has anassociated characteristic acoustic impedance.

According to yet another embodiment of the invention, the ultrasonictransducer further includes a back impedance converter, interposedbetween the backing absorber and the piezoelectric element, wherein themultilayer back impedance converter has a characteristic acousticimpedance between the characteristic acoustic impedances of thepiezoelectric element and the backing absorber.

According to yet another embodiment of the invention, the ultrasonictransducer further includes a quarter wavelength matching layer incontact with and disposed between the propagation medium and the frontacoustic impedance converter. The piezoelectric layer may have an airbacking with this design of a double matching structure to providesufficiently wide bandwidth for certain applications.

Thus, according to an aspect of the invention, a transducer arrangementhaving an impedance converter that is substantially thinner than onequarter wavelength is compensated for by means of a material layerhaving a relatively higher impedance (or higher density material)positioned on the lower impedance side (propagation medium side) of theconverter. In this manner, the material layer compensates for otherwisedegraded converter performance and operates to provide or recoversubstantially the original impedance conversion function. The higherimpedance material layer may comprise a metal layer positioned betweenthe thickness reduced converter layer comprising a polymer layer and thelower impedance region adapted to be converted to a higher impedance.

An impedance conversion layer useful for medical imaging ultrasonictransducers comprises a low impedance polymer layer and a high impedancemetal layer. These layers are combined with corresponding thicknessesadapted to provide a function of converting from a specific highimpedance to specific low impedance, wherein the polymer layer is at thehigh impedance side and the metal layer is at the low impedance side.The effective acoustic impedance of the polymer and metal layercombination may be adapted to configure an impedance converter in thesame way as a quarter wavelength impedance converter, converting fromlow impedance to high impedance (metal to polymer) or from a highimpedance to low impedance (polymer to metal). This structure may beused for front matching with the propagation medium and back matchingwith an absorber for ultrasonic transducers.

BRIEF DESCRIPTION OF THE FIGURES

Understanding of the present invention will be facilitated byconsideration of the following detailed description of the preferredembodiments of the present invention taken in conjunction with theaccompanying drawings, in which like numerals refer to like parts and inwhich:

FIG. 1A is a metal polymer multilayer impedance converter for ultrasonictransducers, according to an embodiment of the invention;

FIG. 1B is a mass and spring model for the metal polymer multilayerimpedance converter of FIG. 1A;

FIG. 2 illustrates the impedance performance of a prior art quarterwavelength matching layer with Z_(R)=1.5 MRayl, loaded with water;

FIG. 3 illustrates the impedance performance of a polyimide/coppermultilayer impedance converter designed for 6.8 MHz, according to anembodiment of the invention;

FIG. 4 illustrates an ultrasonic transducer with a polymer-metalmultilayer impedance converter, according to an embodiment of theinvention;

FIG. 5 illustrates an embodiment of an ultrasonic transducer with amultilayer impedance converter on a front face of a piezoelectric layerand a backing absorber layer, according to an embodiment of theinvention;

FIG. 6 illustrates a simulated performance of the ultrasonic transducerof FIG. 4, according to an embodiment of the invention;

FIG. 7A is a sectional view of an ultrasonic transducer having animpedance converter of the invention, according to an embodiment of theinvention;

FIG. 7B is a sectional view of an ultrasonic transducer having doublematching or impedance converter layers, according to another embodimentof the invention;

FIG. 8 illustrates an experimental observation of an output waveform asa function of time after a short pulse excitation of an ultrasonictransducer of FIG. 7A; and

FIG. 9 illustrates a frequency response curve resulting from a Fouriertransform of the waveform of FIG. 8.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference will now be made to various embodiments of the invention,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts. It is to be understood thatthe figures and descriptions of the present invention have beensimplified to illustrate elements that are relevant for a clearunderstanding of the present invention, while eliminating, for purposesof clarity, many other elements found in typical ultrasonic transducers.Because such elements are well known in the art, and because they do notfacilitate a better understanding of the present invention, a discussionof such elements is not provided herein. The disclosure herein isdirected to all such variations and modifications known to those skilledin the art.

In prior art ultrasonic transducers, a layer of low characteristicimpedance material, with a thickness of one quarter of the wavelength ofthe center frequency of the transducer, is provided between thepiezoelectric element and a propagation medium. Exemplary lowcharacteristic impedance materials for air ultrasonic transducersinclude aerogels and plastic foams. For imaging transducers such asthose useful in medical applications, such low characteristic impedancematerials include substantially pure polymer and/or polymer loaded withpowder and/or fibers. The use of a layer of low characteristic impedancematerial, referred to as a matching layer or an impedance converter,improves the conversion ratio of electric to acoustic energy intransmitting of acoustic signals, as well as preventing or reducingundesirable phase shift, compared to ultrasonic transducers with nomatching layer. However, such matching layers have disadvantages,including an undesirably large thickness for some applications. Inaddition, if the thickness of the matching layer varies from one quarterof a wavelength of the acoustic wave, the conversion ratio decreases,resulting in observable phase shifts. A matching layer thus is generallyundesirable in transducers where broadband or wideband performance(e.g., consistent performance over a wide band of acoustic wavelengths)is required. For example, pulse excitation and reception, often employedin medical ultrasonic imaging, requires good broadband performance.Moreover, as suitable materials are not single phase, scattering ofacoustic energy occurs, resulting in undesirable propagation loss. Stillfurther, it is difficult to manufacture suitable materials to obtainspecific desired characteristic acoustic impedances, resulting inproblems relating to mass production of such transducers for specificapplications.

At least some of the drawbacks associated with prior art transducers areaddressed in an embodiment wherein a transducer includes a piezoelectricelement and a polymer layer disposed on the piezoelectric element. Ametal layer is disposed on the polymer layer. The polymer layer and themetal layer together constitute an impedance converter. The thicknessesof the polymer layer and the metal layer are so selected as to providethe impedance converter with an effective characteristic acousticimpedance intermediate the characteristic acoustic impedances of thepiezoelectric element and of the propagation medium. Advantageously, byselecting the thicknesses of the metal and polymer layer, a range ofeffective characteristic acoustic impedances is available. The thicknessof this impedance converter may be configured to be much less than onequarter of the wavelength of the target frequency of the acousticsignals.

According to an aspect of the invention, the condition wherein theimpedance converter is substantially thinner than one quarter wavelengthis compensated for by means of an additional material layer having arelatively higher impedance (or higher density material) positioned onthe lower impedance side (propagation medium side) of the converter. Inthis manner, the additional layer compensates for otherwise degradedconverter performance and operates to provide (i.e. recovers)substantially the original impedance conversion function. The higherimpedance material layer (e.g. metal layer) is positioned between thethickness reduced converter layer (e.g. polymer layer) and the lowerimpedance region adapted to be converted to a higher impedance.

The ratio of thicknesses of the metal layer and the polymer layer may bedetermined based on a predetermined center resonant frequency of thetransducer and material parameters. By adapting the thicknesses of bothlayers to be thinner than one quarter wavelength in each material, inconjunction with determining the thickness ratio among the layers, themethod and apparatus of the present invention provides the function of aquarter wavelength impedance converter. Further, the present inventionenables the design of arbitrary conversion ratios according to theselection of thickness ratios for each of the layers and thecorresponding layer component materials.

Further, by combining two conventionally available material layers withhigh and low impedances (e.g. a metal layer and a polymer layer), anarbitrary effective acoustic impedance Z_(m) is synthesized as describedherein, having the same function as that of a quarter wavelengthimpedance converter with acoustic impedance Z_(m). The method andapparatus of the present invention thus mitigates the problemsassociated with obtaining specific values of acoustic impedance Z_(m)according to conventional quarter wavelength designs.

Advantageously, an impedance converter having a desired characteristicacoustic impedance can readily be fabricated from commercially availablemetal and polymer materials, thereby facilitating mass production ofimpedance converters and reducing costs of production compared to priorart matching layers. Good performance over a broadband range around thecenter resonant frequency may be obtained, so that a transducer with animpedance converter according to the invention is suitable forapplications, such as medical imaging, requiring good broadbandperformance.

Referring now to FIGS. 1A-1B, an ultrasonic transducer 100 with amultilayer acoustic impedance converter 120 is schematicallyillustrated. Transducer 100 includes a piezoelectric body or element 110having a high characteristic acoustic impedance Z_(p) (e.g., 20-30×10⁶kg/m²s or MRayl). Multilayer acoustic impedance converter 120 includes apolymer layer 130 having a predetermined thickness t_(p) and a metallayer 140 having a predetermined thickness t_(m). In an exemplaryembodiment, the layers 130, 140 each may be of substantially uniformthickness t_(p) and t_(m) respectively. In other configurations, layers130, 140 may have varying thicknesses t_(p) and t_(m) respectively. Theeffective or equivalent characteristic acoustic impedance Z_(C) of thecombined polymer layer 130 and metal layer 140 is adapted to provideappropriate functionality as an acoustic impedance converter 120. In anexemplary embodiment, polymer layer 130 has parallel opposing planarfirst surface 132 and second surface 134. Metal layer 140 has parallelopposing planar first surface 142 and second surface 144. In anexemplary configuration, metal layer 140 may be of copper, brass,aluminum, steel or other suitable metal. First surface 132 of polymerlayer 130 is bonded or otherwise coupled to piezoelectric body orelement 110 of ultrasonic transducer 100. First surface 142 of metallayer 140 is bonded or otherwise coupled (e.g. via electroplating metalonto polymer or via electroless plating) to second surface 134 ofpolymer layer 130. Second surface 144 of metal layer 140 may be disposedin contact with a propagation medium 150 (for example, water or abiological tissue of an animal, such as a human) having a lowcharacteristic acoustic impedance Z_(R) (e.g., 1.5 MRayl). It isunderstood that metal layer 140 may include a thin material coating, forexample, a paint coating, having negligible effect on the effectivecharacteristic acoustic impedance of the converter 120, and interposedbetween metal layer 140 and propagation medium 150. The presence of sucha thin layer is also intended to be interpreted as metal layer 140 beingin contact with propagation medium 150. Metal layer 140 serves totransmit ultrasonic energy between propagation medium 150 andpiezoelectric element 110 through polymer layer 130. The layers 130, 140may be directly in contact with one another at respective surfaces 134,142, or may be coupled via a thin adhesive layer disposed therebetween.The thin adhesive layer has no significant effect (i.e. negligibleeffect) on the effective acoustic impedance of acoustic impedanceconverter 120.

Still referring to FIG. 1A, an ultrasonic wave 160 originates frompiezoelectric element 110 and propagates normal to the surface ofpolymer layer 130 from piezoelectric element 110 towards metal layer 140and into propagation medium 150.

The thicknesses t_(m) and t_(p) of the metal and polymer layers 140, 130respectively, may be selected so that the effective characteristicacoustic impedance Z_(C) of impedance converter 120 is between that ofpropagation medium 150 (i.e., Z_(R)) and the active piezoelectricelement 110 (i.e., Z_(P)).

FIG. 1B illustrates a mass and spring model used to calculate the waveimpedance or specific acoustic impedance Z₂ of acoustic impedanceconverter 120 as seen from the surface 132 of polymer layer 130 to metallayer 140. A mass 170 represents metal layer 140 and a spring 180represents polymer layer 130. Spring 180 has an end point 190. Thefollowing calculations and conditions are provided by way of example toillustrate how impedance conversion takes place by means of the polymerand metal layers. Actual values are not limited to the precise valuesdescribed herein but based on practical requirements for the specificapplication. For example, certain applications may not require a widebandwidth, while in other cases, design of bandwidth and sensitivity areachieved as the cumulative effect of each section (e.g. front and back)without satisfying the below criteria. In any event, for calculations ofspecific acoustic impedance Z₂, the mass M of mass 170 is calculated as:

M=ρ_(m)=ρ_(m)t_(m) per unit area,  (1)

wherein ρ_(m) is the density of metal layer 140; and

-   -   t_(m) is the thickness of metal layer 140.        Likewise, the spring constant K of spring 180 is calculated as:

K=c ₃₃ /t _(p)=ρ_(p) V _(p) ² /t _(p)  (2)

wherein c₃₃ is the stiffness constant in the direction of thicknesst_(p);

-   -   t_(p) is the thickness of polymer layer 130;    -   ρ_(p) is the density of polymer layer 130;    -   V_(p) is the acoustic velocity in polymer layer 130.        Equation (2) uses the known relationship V_(p)=√(c₃₃/ρ_(p)).        In the model above, the mass of polymer layer 130 was neglected.        However, a part of polymer layer 130 proximate to metal layer        140 moves with metal layer 140 such that at least a portion of        the mass of polymer layer 130 influences the mass of metal layer        140. When metal layer 140 is thinner than polymer layer 130, the        mass M of mass 170 may, therefore, be approximated as:

M=ρ _(m) t _(m)+0.4(ρ_(p) t _(p))  (3)

As is known in the art, the resonant frequency f_(o) of impedanceconverter 120 may then be calculated as:

$\begin{matrix}{f_{0} = {\frac{1}{2\pi}\sqrt{\frac{K}{M}}}} & (4)\end{matrix}$

As described below, the specific acoustic impedance at end point 190 ofspring 180 is the highest at a resonance condition. When subjected to anultrasonic wave 160 (see FIG. 1A), end point 190 displaces to the leftin FIG. 1B, i.e., towards mass 170, and spring 180 generates a forcepushing mass 170 to the left, at a given start time. Consequently, mass170 starts to move and is displaced to the left. The displacement ofmass 170 is a maximum after one quarter cycle of the resonant frequency,since the displacement of mass 170 lags by one quarter cycle thedisplacement of the end point 190 of spring 180 and the amplitude of thespring oscillation is greatest at a resonant frequency. At one quartercycle after the start time, spring 180 elongates to a maximum length andthe force from spring 180 is the largest. At this time, end point 190returns to its original position, and the vibration velocity of endpoint 190 reaches its maximum in one cycle period as the mass 170 is nowmoved to the left and exerts force on spring 180.

As is known in the art, the specific acoustic impedance Z₂ of impedanceconverter 120 is given by the force at end point 190 divided by thevelocity at that time. Since at a resonant frequency, the force on endpoint 190 is at a maximum, the specific acoustic impedance Z₂ becomes amaximum at the resonant frequency. In an ideal model, without springlosses, the specific acoustic impedance Z₂ approaches infinity at theresonant frequency and the resonance is sharp. However, the radiation orpropagation medium impedance Z₁ is attached to mass 170 and its effectis equivalent to a resistive load. The propagation medium impedance Z₁thereby serves to damp the resonance. As a result, the resonance isbroadened. An analysis of mass 170 and spring 180 with radiation orpropagation impedance Z₁ at mass M provides the specific acoustic orwave impedance Z₂, at resonant frequency f_(o) as seen from end point190 as:

Z ₂ =MK/Z ₁  (5)

wherein, Z₁=Z_(R)=ρ₁V₁ is the characteristic acoustic impedance ofpropagation medium 150 (e.g., about 1.5 MRayl);

-   -   ρ₁ is the density of propagation medium 150; and    -   V₁ is the acoustic velocity of propagation medium 150.        Thus, using the practical parameters of Z₁=Z_(R) and M (i.e.,        mass per unit area of metal layer 140) and K (i.e., spring force        per unit area divided by displacement for polymer layer 130),        the specific acoustic impedance Z₂ of converter 120 has a much        higher value than the radiation or propagation impedance Z_(R).        This acoustic impedance converter 120 having thinned polymer and        metal layers, has the same function as the well known quarter        wavelength relatively thick matching layer in contact with        propagation medium 150. In the conventional quarter wavelength        matching layer case, the specific wave impedance of the quarter        wavelength layer as seen from the back side is converted to

Z ₂ =Z _(m) ² /Z ₁  (6)

wherein, Z_(m) is the characteristic acoustic impedance of the quarterwavelength matching layer. In the prior art, this quarter wavelengthmatching layer is bonded to the front surface of a piezoelectric layer(having characteristic acoustic impedance Z_(PZT)=30 MRayl) in anultrasonic transducer. Impedance Z₂ is the wave or specific acousticimpedance seen from the piezoelectric layer. Thus, the propagationmedium acoustic impedance Z₁ is up-converted to Z₂, which is close toZ_(PZT).

As is known in the art, for a piezoelectric material having a highcharacteristic acoustic impedance Z_(PZT), the specific acousticimpedance Z₂ of converter 120 has to be close to Z_(PZT) for anefficient energy transfer between the piezoelectric material andimpedance converter 120 and Z₁=Z_(R) (i.e., acoustic impedance ofpropagation medium 150). In an ideal matching condition, if Z₂=Z_(PZT),Z_(m) has to be equal to √(Z_(PZT)Z_(R)). However, as a practicalmatter, the value of the specific acoustic impedance Z₂ of converter 120need not be identical to the value of the characteristic acousticimpedance Z_(PZT) of the active piezoelectric material. In exemplaryembodiments, the specific acoustic impedance Z₂ of converter 120 is notsignificantly different from the value of the characteristic acousticimpedance Z_(PZT) of piezoelectric element 110 and the conditionZ₁<Z_(m)<Z_(PZT) generally holds true. The value of Z_(m) isconventionally chosen to be between Z₁ and Z_(PZT) depending on thedesign requirements for the particular application.

Still referring to FIG. 1A in conjunction with FIG. 1B, the thicknesst_(m) of metal layer 140 and the thickness t_(p) of polymer layer 130may be calculated as follows. Equation (4) above states:

$f_{0} = {\frac{1}{2\pi}\sqrt{\frac{K}{M}}}$

Inserting the values for K and M from Equations (2) and (1) respectivelyin Equation (4),

$\begin{matrix}{f_{0} = {\frac{Vp}{2\pi}\sqrt{\frac{\rho_{p}}{\rho_{m}t_{m}t_{p}}}}} & (7)\end{matrix}$

Further, from Equation (6) above,

Z _(C) ² =Z ₁ ·Z ₂  (8)

and from Equation (5) above,

Z ₁ ·Z ₂ =MK  (9)

Thus, from Equations (8) and (9),

Z _(C)=√(MK)  (10)

This equation means the value Z_(C) may be chosen by selecting materialswith thicknesses that yield suitable values of M and K. The value Z_(C)may be called an effective characteristic acoustic impedance of acousticimpedance converter 120 and provides for selection of an effectivecharacteristic acoustic impedance for a multilayer impedance converter.While the structures associated with the aforementioned cases aredistinct, the effect of the impedance conversion is the same. Theimpedance Z₁ is converted to Z₂ and the multilayer converter structurehas its effective acoustic impedance Z_(C) as Z_(m). If the conversionratio Z₂/Z₁ is the same for both cases, then Z_(C) corresponds to Z_(m),thereby being equivalent in function.Inserting values of M and K from Equations (1) and (2) respectively intoEquation (10), there is obtained

Z _(C) =V _(p)·√(ρ_(m)·ρ_(p) ·t _(m) /t _(p))  (11)

Equations (7) and (11) can be solved for t_(m) and t_(p) as follows:Equation (7) is first solved for V_(p) and the value of V_(p) issubstituted into Equation (11). t_(m) may then be determined as below:

t _(m) =Z _(C)/(ρ_(m)2πf _(o))  (12)

The thickness t_(m) of metal layer 140 is linearly dependent on thedesired effective characteristic acoustic impedance Z_(C) of impedanceconverter 120, and is inversely dependent on the density of the metal ofmetal layer 140 and the center resonant frequency of transducer 100.Equations (7) and (11) are solved by eliminating the term ρ_(m)t_(m) bymaking a product of terms of left side of Equations (7) and (11) to getf₀Z_(C), and by making a product of terms of right side of these twoequations to get Vp²ρ_(p)/t_(p). From equality of the left and rightproducts, we get t_(p) as below:

t _(p) =V _(p) ²ρ_(p)/(2πf ₀ Z _(C))  (13)

The thickness t_(p) of polymer layer 130 is inversely dependent on thecenter resonant frequency f_(o) of transducer 100 and the desiredeffective characteristic acoustic impedance Z_(C) of impedance converter120. The thickness t_(p) of polymer layer 130 is directly linearlydependent on the density of the polymer of polymer layer 130. Thethickness t_(p) of polymer layer 130 is further proportional to thesquare of the acoustic velocity in the polymer layer 130. Thus, for agiven or required Z_(C) for a given application and a given centerresonant frequency f_(o), thickness t_(m) of metal layer 140 andthickness t_(p) of polymer layer 130 may be calculated using Equations(12) and (13). Both thicknesses t_(m) and t_(p) are linearly related tothe center resonant frequency f_(o) of transducer 100. The ratio of thethickness t_(m) of metal layer 140 to the thickness t_(p) of polymerlayer 130 may be expressed as

t _(m) /t _(p) =Z _(C) ²/(ρ_(m) V _(p) ²·ρ_(p))  (14)

The thickness ratio is accordingly independent of the center resonantfrequency f_(o) of transducer 100. The ratio of the metal thicknesst_(m) to the polymer thickness t_(p) increases with the square of thedesired effective characteristic acoustic impedance Z_(C) of impedanceconverter 120. It will be understood that the values of thicknessest_(m) and t_(p) calculated using Equations (12) and (13) may serve asstarting points for the design of acoustic impedance converter 120 andmay be varied therefrom without departing from the scope of theinvention. The thicknesses t_(m) and t_(p) may be varied depending onthe commercial availability of the chosen materials of standardthicknesses. These variations in the thicknesses of t_(m) and t_(p) fromthose determined through Equations (12) and (13) are intended to bewithin the scope of the present invention.

It will be further understood that an acoustic impedance converter mayperform satisfactorily even though the thicknesses t_(m) and t_(p) maynot satisfy Equations (12) and (13). A desired overall performance foran ultrasonic transducer may be achieved with a non-ideal front acousticimpedance converter and a non-ideal back impedance converter, both ofwhich may deviate from the values determined using the method describedherein. However, the phase shift resulting from the front and backmatching layer(s) may be cancelled by using a higher resonant frequencyfor the front matching layer(s) and a lower resonant frequency for theback matching layer(s) relative to the center resonant frequency. Yetanother example is an ultrasonic transducer with no back matching layer(i.e., with air backing), which may use double front acoustic impedanceconverters in order to provide a structure with sufficiently widebandwidth for a given application. In such a transducer each individualacoustic converter may deviate from the ideal values. However, theeffective combined characteristic acoustic impedance may providesatisfactory overall performance because of the cancelling effect of thetwo acoustic impedance converter structures, wherein one of the acousticimpedance converter may be configured for a higher resonant frequencyand the other for a lower resonant frequency relative to the centerresonant frequency of ultrasonic transducer 100.

Referring now to FIG. 2, there is illustrated a chart 200 depicting theresults of a specific acoustic impedance Z₂ calculation using a onedimensional wave propagation model (i.e., not using the mass and springmodel), as known in the art, for wave or specific acoustic impedance Z₂of a quarter wavelength matching layer for which the characteristicacoustic impedance has an ideal value of Z_(m)=6.7×10⁶ Rayl andZ_(R)=1.5×10⁶ Rayl for the propagation medium loaded at the front sideof the quarter wavelength matching impedance layer of an ultrasonictransducer. The wave impedance 210 seen from the back of the quarterwavelength matching layer shows a peak at 6.8 MHz with a value ofspecific acoustic impedance Z₂=30×10⁶ Rayl which is equal to thecharacteristic acoustic impedance of the piezoelectric material,Z_(PZT). However, as set forth above, it is difficult to obtain amaterial having the desired ideal value of characteristic acousticimpedance Z_(m)=6.7×10⁶ Rayl for fabricating the quarter wavelengthmatching layer of an ultrasonic transducer.

Referring now to FIG. 3, a chart 300 illustrates the specific acousticimpedance Z₂ seen from polymer layer 130 to propagation medium 150 for amultilayer impedance converter 120 of FIG. 1A calculated using the samerigorous one dimensional wave propagation model as for FIG. 2. In anexemplary configuration, polymer layer 130 takes the form of a polyimidelayer with density ρ_(p)=1454 kg/m³, sound velocity V_(p)=2175 m/s andthickness t_(p)=20 μm (about 1/16 of the wavelength). In thisembodiment, metal layer 140 takes the form of a copper layer 12 withdensity ρ_(m)=8960 kg/m³, sound velocity V_(m)=5010 m/s and thicknesst_(m)=19.7 μm (about 1/37 of the wavelength). This exemplaryconfiguration of impedance converter 120 yields specific acousticimpedance Z₂=30 MRayl at 6.8 MHz peak which ideally matches tocharacteristic acoustic impedance Z_(PZT) of bulk PZT which typicallyhas Z_(PZT)=30 MRayl. It is further noted here that the quarterwavelength in polyimide layer 130 is 80 μm and the copper layer is 184μm at 6.8 MHz and, therefore, 20 μm as per the present design is muchthinner than a quarter of the wavelength at the center resonantfrequency of the piezoelectric active material. It is understood thatthe design of the same impedance conversion ratio at a differentfrequency can be accomplished using the same ratio of thickness towavelength for the same materials.

The impedance curve 310 of impedance converter 120 (of FIG. 1A) shown inFIG. 3 is very close to that of the ideal impedance performance depictedby curve 210 shown in FIG. 2. Thus, selection of suitable thicknessesand material parameters of the polymer 130 and metal 140 layers providesfor an arbitrary value of an equivalent characteristic acousticimpedance. It is noted that the thicknesses of each of polymer layer 130and metal layer 140 are much thinner than the quarter wavelengthmatching layers of the same materials.

Referring now to FIG. 4, there is schematically illustrated anultrasonic transducer 400 configured to operate at high frequency (i.e.,in the MHz region) according to an embodiment of the invention.Transducer 400 includes a piezoelectric element 110 having a frontacoustic impedance converter 120 bonded to piezoelectric element 110 ata boundary plane 112. A backing absorber 410 is bonded to thepiezoelectric element 110 at a boundary plane 114. Impedance converter120 may be bonded to piezoelectric element 110 using an adhesive such asan epoxy. Boundary plane 114 is opposite to boundary plane 112. Thus,backing absorber 410 is attached to piezoelectric element 110 on a faceopposite the face of piezoelectric element 110 on which is attachedfront impedance converter 120. Front acoustic impedance converter 120contacts propagation medium 150. It will be understood that frontacoustic impedance converter 120 may contact propagation medium 150through a thin intervening layer, such as a paint coating, having anegligible effect on the effective characteristic acoustic impedance ofconverter 120.

Generally, piezoelectric element 110 (for example, a piezoelectricceramic layer) has a high characteristic acoustic impedance Z₁ (about20-30 MRayl depending on the configuration and the material, e.g.,Z_(PZT) approximately equal to 30 MRayl)). Propagation medium 150generally has a relatively low characteristic acoustic impedance Z_(R)(for example, about 1.5 MRayl). Acoustic impedance converter 120includes a polymer layer 130 of thickness t_(p) and a metal layer 140 ofthickness t_(m) bonded to polymer layer 130. The thicknesses t_(m) andt_(p) for the metal layer 140 and the polymer layer 130 have beenselected based on the desired or predetermined equivalent or effectivecharacteristic acoustic impedance Z_(C) of acoustic impedance converter120. An ideal value of specific acoustic impedance Z₂, (which, as notedabove is close to characteristic acoustic impedance Z_(PZT)) determinedby the effective characteristic acoustic impedance Z_(C) can be obtainedas shown in FIG. 3. Thus, a wideband transducer can be configured moreeasily and economically than the prior art systems and methods.

Generally, the vibration of piezoelectric element 110 excites acousticwaves in a forward direction (to the left in FIG. 4) toward propagationmedium 150 and also in a rearward direction (to the right in FIG. 4)toward backing absorber 410. Some of the energy of acoustic wavespropagating in a rearward direction is transmitted to backing absorber410. The remainder is reflected at back boundary 414 betweenpiezoelectric element 110 and backing absorber 410. A certain amount ofreflection is desirable to enhance the sensitivity of transducer 400 todrive and to receive ultrasonic waves. This reflection at back boundary414 is a function of the characteristic acoustic impedance of backingabsorber 410. If the acoustic absorption in backing absorber 410 is notsufficiently high, ultrasonic waves reflected from an end surface 412 ofbacking absorber 410 may be reflected back to piezoelectric element 110and may overlap with a front wave propagating towards propagation medium150. Disadvantageously, the ultrasonic waves may be destructively orconstructively added depending on the frequency, and multiple resonancepeaks may be formed on the frequency response curve of transducer 400.Another problem caused by excessively low acoustic absorption by theabsorption material of backing absorber 410 is that the thickness ofbacking absorber 410 has to be increased so as to absorb substantiallyall backward waves and prevent reflection from end surface 412. If thethickness of backing absorber 410 is excessive, then the transducer 400,with its backing absorber, may not fit within the limitations of thetransducer holder or housing, the size of which may be constraineddepending on the application of transducer 400.

FIG. 5 depicts an ultrasonic transducer 500 having a back impedanceconverter 560 for backing absorber 410 according to another embodimentof the invention. Generally, characteristic acoustic impedance (MRayl)and acoustic absorption (decibel/centimeter or dB/cm) are closelyrelated and cannot be controlled independently. A material having a highacoustic absorption often has a low characteristic acoustic impedance.The desired characteristic acoustic impedance of a backing absorber 410may vary depending on the material and structure of the activepiezoelectric layer. Examples of the active piezoelectric layer havingvarious desirable characteristic acoustic impedances for backingabsorber 410 include bulk PZT, 2-2 composite, 1-3 composite and singlecrystals.

As shown in the exploded view of FIG. 5, transducer 500 includes aPZT/polymer 2-2 connectivity composite array 520, a front acousticimpedance converter 120 attached (e.g. bonded) to a front surface 526 ofarray 520 via a grounding layer 510, a flexible printed circuit board540 with conductor traces 530 (e.g., copper), a shielding conductivelayer 550 (e.g. copper) and a backing absorber 410. Backing absorber 410is attached (e.g. bonded) to a back surface 528 of array 520 throughconductive shielding layer 550, flexible circuit board 540 and conductortraces 530. Flexible printed circuit board 540 serves to provide a routefor electrical signals and also functions as a back acoustic impedanceconverter 560 including the polymer of the flexible printed circuitboard 540 and the metal of shielding conductive layer 550 to up-convertthe low characteristic acoustic impedance of backing absorber 410.Composite array 520 includes multiple narrow elongated elements 524 (forexample, about 10 millimeters (mm)×0.1 mm) of PZT with kerfs or channels522 (for example, of about 50 micrometers (μm) width) therebetweenfilled with a polymer, such as epoxy. Each piezoelectric element 524 ofcomposite array 520 may be driven with different signals havingdifferent phases to steer beam direction. Backside electrodes (notshown) of composite array 520 are connected to conductive traces 530 offlexible printed circuit board 540, along a first surface 542 offlexible printed circuit board 540. The flexible printed circuit board540 is coupled along a second surface 544 thereof, opposite to firstsurface 542, to back acoustic impedance converter 560 which, in turn, iscoupled to a backing absorber 410. Because of the narrow width geometryof piezoelectric elements 524, the characteristic acoustic impedanceZ_(PZT) of piezoelectric elements 524 may be as low as 15 MRayl. In thisembodiment, acoustic impedance converter 120 has metal layer 140 ofthickness t_(m)=15 μm as a front layer and polymer layer 130 ofthickness t_(p)=40 μm on metal layer 140. This exemplary metal-polymermultilayer acoustic impedance converter 120 yields specific acousticimpedance Z₂=15 MRayl at 5.2 MHz, which Z₂ matches the Z_(PZT) of 2-2PZT/polymer composite array 520.

In the illustrated embodiment of FIG. 5, polymer layer 130 of acousticimpedance converter 120 may be of polyimide and metal layer 140 may beof copper. The thickness of copper layer 140 may be so selected as toprovide an appropriate acoustic impedance conversion from the lowcharacteristic acoustic impedance of propagation medium 150 (Z_(R)) tothe high characteristic acoustic impedance Z_(PZT) of PZT/polymercomposite array 520. A second copper layer 510 is interposed betweenacoustic impedance converter 120 and PZT/polymer array 520 as aconnection to ground. Copper layer 510, however, does not function as anacoustic impedance converter because layer 510 has a characteristicacoustic impedance similar to that of piezoelectric elements 524 andbecause layer 510 is directly bonded to piezoelectric elements 524 as agrounded electrode. Therefore, the presence of copper layer 510 does notinfluence the design of composite or multilayer acoustic impedanceconverter 120. It will be understood that PZT composite array 520, frontmatching or acoustic impedance converter 120 and back acoustic impedanceconverter 560 are shown separately (i.e., not bonded or otherwisecoupled) for illustrative purposes only.

To provide backing absorber 410 with an appropriate acoustic impedanceconversion, back acoustic impedance converter 560 in FIG. 5 may beprovided between piezoelectric array 520 and backing absorber 410.Generally, the characteristic acoustic impedance of backing absorber 410is about 4-10 MRayl, which is higher than the characteristic acousticimpedance of front propagation medium 150, which may be about 1.5 MRayl,for example. The desired effective acoustic impedance Z_(C) of backacoustic impedance converter 560 may be selected to be consistent withthe desired bandwidth and sensitivity of transducer 500. For example,when the characteristic acoustic impedance of backing absorber 410 is 5MRayl, and a required specific backing acoustic impedance is 10 MRayl,the desired effective characteristic impedance of the back acousticimpedance converter Z_(C)(=√(MK)=√(Z₁·Z₂)) is 7.07 MRayl. This value ofthe effective characteristic acoustic impedance of back acousticimpedance converter 560 is obtained by using a metal (for example,copper) layer 550 with thickness t_(m)=24 μm and a polymer (for example,polyimide) layer 540 with thickness t_(p)=29 μm, for a transducer havinga center resonant frequency of 5.2 MHz. The selection of appropriatematerials and thicknesses t_(m), t_(p) for metal layer 550 and polymerlayer 540 for back acoustic impedance converter 560 interposed betweenan active piezoelectric element 524 and a backing absorber 410 is madein substantially the same manner as for front acoustic impedanceconverter 120 between active piezoelectric array 520 and frontpropagation medium 150. Back acoustic impedance converter 560 convertsthe low characteristic acoustic impedance Z₁ of backing absorber 410 toa higher specific acoustic impedance Z₂ which is the wave impedance orspecific impedance as seen from active piezoelectric array 520 to theinterior of backing absorber 410. An appropriate value for specificacoustic impedance Z₂ is determined from the desired bandwidth andsensitivity of transducer 500. The desired value of the effectivecharacteristic acoustic impedance, Z_(C), of back acoustic impedanceconverter 560 is calculated using the equation Z_(C)=square root of theproduct of Z₂ and Z₁ (i.e. Z_(C)=√(Z₂×Z₁)). The thickness t_(m) of metallayer 550 is determined based on the desired effective characteristicacoustic impedance Z_(C) of back acoustic impedance converter 560, thedensity of the metal of metal layer 140, and the center resonantfrequency f_(o) of transducer 500. The thickness t_(p) of polymer layer540 is calculated based on the desired effective characteristic acousticimpedance Z_(C) of back acoustic impedance converter 560, the density ofthe polymer of polymer layer 540, the acoustic velocity in the polymerof polymer layer 540, and the center resonant frequency f_(o) oftransducer 500.

Table I below lists the material parameters for an exemplary propagationmedium (water), an exemplary piezoelectric active material (PZT), anexemplary metal (copper), a polyimide and Polyvinylidene fluoride(PVDF). As is known in the art, the characteristic acoustic impedance ofa material is given by the product of the density of the material andthe velocity of sound in the material.

TABLE I Material parameters used for design of various examples in TableII Propagation PVDF medium PZT-5H Copper Polyimide (for matching)Density 1000 7500 8960 1454 1780 (kg/m³) Velocity 1500 4800 5010 21752100 (m/s)

The following Table II compares the two calculated values of specificacoustic impedances Z₂, one calculated by a mass and spring model andthe other calculated by a rigorous one dimensional model for low andhigh values (15 and 30 MRayl) of Z_(P), where the value for thecharacteristic acoustic impedance of the propagation medium Z₁=1.5 MRaylwas used. Table II shows that specific acoustic impedance Z₂ calculatedusing the mass and spring model is close enough for actual use.

TABLE II Examples of designed acoustic impedance converters to match low(~15 MRayl) and high (~30 MRayl) characteristic acoustic impedances ofpiezoelectric layers, with a propagation medium, such as water or humanmuscle, having a characteristic acoustic impedance of Z_(R) = 1.5 MRayl.fo = 1/2π Copper Polymer Z₂, √(K/M) thickness: t_(m) thickness: t_(p)M-K model Z₂ 1-D model Materials (MHz) (μm, t_(m)/λ) (μm, t_(p)/λ)(MRayl) (MRayl) Cu-polyimide 2.6 32.8, 0.017 82, 0.098 14.1 15.8Cu-polyimide 2.6 54.4, 0.028 54, 0.065 28.8 30 Cu-polyimide 5.2 16.4,0.017 41, 0.098 14.1 15.8 Cu-polyimide 5.2 27.2, 0.028 27.0, 0.065  28.8 30 Cu-polyimide 10.4  8.4, 0.017 20.5, 0.098   14.1 15.8Cu-polyimide 10.4 13.6, 0.028 13.5, 0.065   28.8 30 Cu-PVDF 5.2 14.3,0.015 49, 0.12  13.0 15.5 Cu-PVDF 5.2 26.3, 0.027 31, 0.078 28.9 30.1

As seen in Table II, when the materials and desired specific acousticimpedance are selected, the thickness ratio of polymer layer 130 tometal layer 140 is generally constant for any given frequency. Forexample, for copper and polyimide, when Z₂=30 MRayl is selected, aseries of f_(o)=2.6 MHz, 5.2 MHz, and 10.4 MHz gives the same thicknessratio of copper/polyimide ≈1, consistent with Equation (14) above. If apolymer other than polyimide is used, the density and acoustic velocitymay differ from that of polyimide, such that the thickness ratio willdiffer, as may be appreciated from Equation (14) above.

Referring now to FIG. 6, there is shown a plot 600 of signal strength,represented as pressure as a function of frequency (Hz) for a simulatedultrasonic transducer in transmitter mode, using a one dimensional model(either the Mason model or the KLM model yield effectively the sameresult). Backing absorber 410 (see FIG. 4 or 5) has a low characteristicacoustic impedance of 4.5 MRayl. Piezoelectric element 110 (see FIG. 4)has a high characteristic acoustic impedance of 30 MRayl; acousticimpedance converter 120 is composed of a polymer layer 130 of 33 μmpolyimide and a metal layer 140 of copper with various thicknesses asshown in FIG. 6. The resonant frequency is f_(o)≈5.2 MHz and thethickness of PZT is 350 μm. Curves 610, 620, 630 and 640 represent thesignal strength for thicknesses of 0 μm, 10 μm, 20 μm and 30 μm formetal layer 140 of copper. A separate calculation, according to themethod described hereinabove, shows that the combination of a 33 μmpolyimide polymer layer 130 and a 20 copper metal layer 140 yields aneffective characteristic acoustic impedance Z_(C) of 6.1 MRayl, and 1.5MRayl of the front propagation medium and is converted to a specificacoustic impedance of 25 MRayl at the front face of PZT layer. Thisspecific acoustic impedance is somewhat lower than the characteristicimpedance of PZT. However, for both high sensitivity and wide bandwidthpurposes, such a slightly lower impedance at the front face of the PZTlayer may be advantageous. For example, FIG. 6 shows curve 630 for 20 μmcopper having high sensitivity and wideband and symmetric frequencyresponse, with a center frequency of 5.2 MHz. The structure of 20 μmmetal layer 140 of copper and 33 μm polymer layer 130 of polyimidecorresponds to a hypothetic material with characteristic acousticimpedance Z_(m)=6.1 MRayl with one quarter wavelength thickness. If thedesign is altered to obtain a different designed center frequency f₀,the PZT thickness is also different and is inversely proportional to thefrequency f₀. It will be understood that the thicknesses t_(m) and t_(p)are also different for a different center resonant frequency f₀. Forsuch a different frequency design, the values of t_(m)/λ and t_(p)/λ maystill be kept constant as can be seen in Table II.

FIG. 6 illustrates a simulation of an acoustic output of an ultrasonictransducer 400 (of FIG. 4) acting as a transmitter. The same multilayeracoustic impedance converter structure 120 functions also for anultrasonic transducer acting as a receiver. The reciprocity principlefor an acoustic device generally states that a given structure of atransducer should have the same bandwidth as a receiver as that as atransmitter. Therefore, multilayer acoustic impedance converter 120described herein may be advantageously used in medical ultrasonicimaging systems. For example, transducer 400 (of FIG. 4) transmitsacoustic beams in a short pulse into a human body which beams are thenreflected from a target material such as a human organ. The acousticbeams are scanned and the reflections are received by the sametransducer 400 (of FIG. 4) and analyzed to reconstruct the image of thehuman organ, for example, for display on a display device. For suchpurposes, transducer 400 (FIG. 4) is used as both transmitter andreceiver; use of the acoustic impedance converter disclosed hereinprovides good wideband performance. Metal layer 140 (FIG. 4) serves totransmit ultrasonic energy generated by piezoelectric element 110 (FIG.4) to propagation medium 150 (FIG. 4) through polymer layer 130 (FIG.4). Metal layer 140 (FIG. 4) also serves to transmit/receive ultrasonicenergy reflected from the target material in propagation medium 150(FIG. 4) to piezoelectric element 110 (FIG. 4) through polymer layer 130(FIG. 4).

Referring now to FIG. 7A, another exemplary embodiment of an ultrasonictransducer 700 with acoustic impedance converter 120 according to theinvention is illustrated. Ultrasonic transducer 700 has an activepiezoelectric element 110 of suitable material, such as PZT. Activepiezoelectric element 110 has front and back planar parallel faces.Backing absorber 747 may be formed from any low characteristic acousticimpedance material, such as air, water or an absorber material. Goodwideband frequency response may be achieved if acoustic impedanceconverter 120 is appropriately designed as described herein. Frontpropagation medium 150 may be water or a human body. Acoustic impedanceconverter 120 has a polymer layer 130 bonded to the front face of activepiezoelectric layer 110, and a metal layer 140 bonded to polymer layer130. In an exemplary embodiment, both layers 130, 140 are ofsubstantially uniform thicknesses t_(p), t_(m) respectively, and theselayers function as an impedance converter as already described. Thepolymer and metal layers are configured so as to have an effectiveacoustic impedance between that of the piezoelectric material and thepropagation medium. The equivalent or effective characteristic acousticimpedance of the multilayer impedance converter 120 is Z₂ and theintermediate characteristic acoustic impedance is Z₂<Z_(PZT). Matchinglayer 710 may be a polymer layer having with a one quarter wavelengththickness and an acoustic impedance Z_(m1) bonded to a front face ofacoustic impedance converter 120, for facing propagation medium 150. Thecharacteristic acoustic impedances of the quarter wavelength matchinglayer 710 and acoustic impedance converter 120 are chosen to satisfy therelationship Z₁<Z_(m1)<Z_(C2)<Z_(PZT).

Referring now to FIG. 7B, another embodiment of the present invention isillustrated wherein first and second sets of multilayer acousticimpedance converters 120 and 743, respectively, are configured as doublematching layers for a transducer 800. Outer acoustic impedance converter743 is configured according to the structure of multilayer acousticimpedance converter 120 as described herein. A lower characteristicacoustic impedance layer 744 (such as a rubber or latex layer) is bondedto metal layer 140 of inner acoustic impedance converter 120 and ahigher characteristic acoustic impedance layer 745 (e.g. a metal,polymer or plastic layer) is adapted to be in contact with propagationmedium 150. Outer multilayer acoustic impedance converter 743 functionsas a quarter wavelength matching layer 710 as shown in FIG. 7A, and theequivalent characteristic acoustic impedance Z_(C1) and Z_(C2) are sochosen as to satisfy the relationship Z₁<Z_(C1)<Z_(C2)<Z_(PZT). Inneracoustic impedance converter 120 has a polymer inner layer 130 bonded topiezoelectric element 110, and an outer metal layer 140 bonded topolymer inner layer 130.

Double matching layers have been utilized in prior art ultrasonictransducers using air as a backing absorber. The first layer disposeddirectly on a PZT element is a quarter wavelength matching layer ofmaterial having a high characteristic acoustic impedance, whichcharacteristic acoustic impedance is lower than that of PZT. The secondlayer is disposed between the first layer and the propagation medium(e.g., water). The high characteristic acoustic impedance material ofthe first layer in prior art air backing transducers consists of a thinlayer of glass (or composite material). The required thickness of such alayer is, for example, 0.52 mm for a transducer having a 2.6 MHz centeroperating frequency, or 0.26 mm thickness for a transducer having a 5.4MHz center operating frequency. However, such thin layers of materialhave proven difficult to manufacture in large scale production systems.The double matching layer apparatus and method as described herein makesit possible to obtain a desired wideband performance that is capable ofsuch large scale manufacturing.

Experimental testing of the embodiment shown in FIG. 7A was accomplishedwherein a uniform PZT plate of 0.85 millimeter (mm) thickness with acenter operating frequency of 2.6 MHz was used as piezoelectric element110. Polymer layer 130 comprised a 33 μm PVDF layer. Metal layer 140comprised a 77 μm layer of brass. It is understood that for suchapplications the acoustic properties of brass are very similar to thoseof copper. Matching layer 710 was a 220 μm polyimide layer. The layerswere bonded by low viscosity, negligibly thin epoxy bonding layer.

FIG. 8 shows operational results associated with transducer 700 of FIG.7A. When the transducer 700 of FIG. 7A is driven by a sharp pulse, theobserved waveform 810 is shown in chart 800 of FIG. 8. The frequencyperformance of the Fourier transformed spectrum 910 of the waveform 810of FIG. 8 is illustrated in chart 900 of FIG. 9. As can be appreciatedfrom FIG. 8, a sharp pulse waveform 810 is obtained. As can be seen bycurve 910 in FIG. 9, wide frequency performance (about 56% relativebandwidth) was achieved.

As previously described, the thicknesses t_(p) and t_(m) of polymerlayer 130 and metal 140 of acoustic impedance converter 120 may bevaried from the values determined using Equations (12) and (13) withoutdeparting from the scope of the invention. At a constant center resonantfrequency, as the thickness t_(p) of polymer layer 130 may be increasedfrom the theoretical value obtained using Equation (13), and thethickness t_(m) of metal layer 140 may be correspondingly decreased fromthe theoretical value obtained using Equation (12). As the thicknesst_(m) of metal layer 140 approaches zero, the thickness t_(p) of thepolymer layer 130 approaches the thickness of a conventional quarterwavelength matching layer. Thus, as the thickness t_(p) of polymer layer130 is decreased from the thickness of a quarter wavelength matchinglayer, the deviation in the resonance response due to a thinner polymerlayer 130 may be compensated by adding metal layer 140 of a giventhickness t_(m). The thickness t_(p) of polymer layer 130 may bedecreased to one-tenth of the theoretical value determined usingEquation (13) and still provide adequate transducer performancedepending on the requirements of a given application.

It is to be understood that when thicknesses deviate from their originalvalues, the impedances Z_(C) are different and the function of impedanceconversion of the layer pair is likewise different. However, otherlayers, such as the backing absorber converter and the outermostconverter layer (in the case of double layer matching) similarlyinfluence the performance and design these layers to compensate thedifference to satisfy overall performance. Thus, in one embodiment, thethickness t_(p) of polymer layer 130 may range from between aboutone-tenth of the theoretical value determined using Equation (13) andless than the thickness of a conventional quarter wavelength matchinglayer for a given center resonant frequency of the transducer. It willbe appreciated that alternative embodiments may have thickness t_(p) ofpolymer layer 130 about two-tenth, three-tenth, four-tenth, and so on,of the theoretical value determined using Equation (13) and beadvantageously employed in different applications requiring differenttransducer performances. It will further be appreciated that alternativeembodiments may also have thicknesses t_(p) of polymer layer 130 of 1.1,1.2, 1.3 (etc.) times the theoretical value determined using Equation(13) for different applications.

In other embodiments, only one of the thicknesses t_(p) and t_(m) may bevaried from the values determined using Equations (12) and (13). Forexample, the thickness t_(p) of polymer layer 130 may be half of thetheoretical value obtained using Equation (13), without changing thecorresponding theoretical thickness t_(m) of metal layer 140. As will beunderstood by one skilled in the art, such a combination would result inthe resonant frequency of acoustic impedance converter 120 beingincreased by a factor of approximately the square root of two (2) (i.e.,1.414) from the predetermined center resonant frequency of thetransducer. The resulting deformation in the response curve may beuseful in other applications of a special frequency response, includingbut not limited to nondestructive evaluation using ultrasound energy andDoppler flow speed detection.

Variations and modifications to the disclosed embodiments are within thescope of the invention. For example, while the piezoelectric units aregenerally shown as relatively thin and flat layers, other shapes andforms may be employed. Surfaces that are disclosed as being on and incontact with one another may have interposed therebetween thin layers ofmaterials such as adhesives having little or no effect on the acousticimpedance of the structure.

While the foregoing invention has been described with reference to theabove embodiments, various modifications and changes can be made withoutdeparting from the spirit of the invention. Accordingly, all suchmodifications and changes are considered to be within the scope of theappended claims.

1. An ultrasonic transducer comprising: a piezoelectric element having a characteristic acoustic impedance; a front acoustic impedance converter coupled to said piezoelectric element, said front acoustic impedance converter comprising: a front polymer layer having a thickness t_(p1) coupled to said piezoelectric element; and a front metal layer for transmitting acoustic energy between said front polymer layer and a propagation medium having a characteristic acoustic impedance, said front metal layer having a thickness t_(m1) and being coupled to said front polymer layer, wherein said characteristic acoustic impedance of said propagation medium is lower than said characteristic acoustic impedance of said piezoelectric element, and wherein said front acoustic impedance converter has an effective characteristic acoustic impedance Z_(C) between said piezoelectric element and said propagation medium characteristic acoustic impedances.
 2. The ultrasonic transducer of claim 1, wherein said thicknesses t_(m1) and t_(p1) are selected based on the densities of said front metal layer and said front polymer layer, said effective characteristic acoustic impedance Z_(C), a predetermined center resonant frequency of said ultrasonic transducer, and the velocity of sound in said front polymer layer.
 3. The ultrasonic transducer of claim 2, wherein said thickness t_(p1) of said front polymer layer is less than one quarter of the wavelength of said predetermined center resonant frequency.
 4. The ultrasonic transducer of claim 2, wherein said thicknesses t_(m1) and t_(p1) are determined according to: t _(m1) =Z _(C)/(ρ_(m)2πf _(o)); and t _(p1) =V _(p) ²ρ_(p)/(2πf _(o) Z _(C)), wherein, Z_(C) is said effective characteristic acoustic impedance of said front acoustic impedance converter, ρ_(m) is the density of the front metal layer, f_(o) is the predetermined center resonant frequency, V_(p) is the velocity of sound in the front polymer layer, and ρ_(p) is the density of the front polymer layer.
 5. The ultrasonic transducer of claim 4, wherein said thickness t_(p1) is at least one-tenth of the value as determined by the equation in claim 4 and less than the thickness of a quarter wavelength matching layer for said predetermined center resonant frequency.
 6. The ultrasonic transducer of claim 2, further comprising a backing absorber coupled to said piezoelectric element, said backing absorber having a characteristic acoustic impedance.
 7. The ultrasonic transducer of claim 6, further comprising a back acoustic impedance converter interposed between said backing absorber and said piezoelectric element, wherein said back acoustic impedance converter comprises: a back polymer layer having a thickness t_(p2) and having first and second surfaces, said first surface of said back polymer layer directly coupled to a metal layer adapted as a signal trace for feeding current to said piezoelectric element; a back metal shielding layer having a thickness t_(m2) and having first and second surfaces, said first surface of said back metal shielding layer coupled to said second surface of said back polymer layer, and said second surface of said back metal shielding layer coupled to said backing absorber, wherein said characteristic acoustic impedance of said back polymer layer is lower than said characteristic acoustic impedance of said piezoelectric element, and wherein said back acoustic impedance converter has an effective characteristic acoustic impedance between said piezoelectric element and said back polymer layer characteristic acoustic impedances.
 8. The ultrasonic transducer of claim 7, wherein said front acoustic impedance converter has a resonant frequency, and said back acoustic impedance converter has a resonant frequency, wherein said resonant frequency of the front acoustic impedance converter is higher than the predetermined center resonant frequency of the ultrasonic transducer, and wherein said resonant frequency of the back acoustic impedance converter is lower than the predetermined center resonant frequency of the ultrasonic transducer.
 9. The ultrasonic transducer of claim 1, further comprising a quarter wavelength matching layer for being in contact with said propagation medium, said quarter wavelength matching layer coupled to said front metal layer.
 10. The ultrasonic transducer of claim 1, further comprising a high characteristic acoustic impedance layer for being in contact with said propagation medium and a low characteristic acoustic impedance layer coupled to said front metal layer.
 11. The ultrasonic transducer of claim 10, wherein said high characteristic acoustic impedance layer comprises at least one of a metal and a high characteristic acoustic impedance polymer.
 12. The ultrasonic transducer of claim 11, wherein said high characteristic acoustic impedance polymer comprises at least one of a polyimide and polyester.
 13. The ultrasonic transducer of claim 10, wherein said low characteristic acoustic impedance layer comprises at least one of rubber and latex.
 14. The ultrasonic transducer of claim 10, further comprising an air backing for said piezoelectric element, said air backing being on a side of said piezoelectric element opposite to said front polymer layer.
 15. An ultrasonic transducer comprising: a piezoelectric body having a characteristic acoustic impedance and having first and second surfaces; and an acoustic impedance converter coupled to said first surface of said piezoelectric body, said acoustic impedance converter comprising: a polymer layer having a thickness t_(p1) and having first and second surfaces, said first surface of said polymer layer being coupled to said first surface of said piezoelectric body; and a metal layer for being in contact with a propagation medium having a characteristic acoustic impedance, said metal layer having a thickness t_(m1) and having first and second surfaces, said first surface of said metal layer being coupled to said second surface of said polymer layer, wherein said characteristic acoustic impedance of said piezoelectric body is higher than said characteristic acoustic impedance of said propagation medium, wherein said acoustic impedance converter has an effective characteristic acoustic impedance Z_(C) between said piezoelectric element and said propagation medium characteristic acoustic impedances, and wherein said thicknesses t_(m1) and t_(p1) are selected based on the densities of said metal layer and said polymer layer, said effective characteristic acoustic impedance Z_(C), a center resonant frequency of said ultrasonic transducer and the velocity of sound in said polymer layer.
 16. The ultrasonic transducer of claim 15, wherein said thicknesses t_(m1) and t_(p1) are substantially: t _(m1) =Z _(C)/(ρ_(m)2πf _(o)); and t _(p1) =V _(p) ²ρ_(p)/(2πf _(o) Z _(C)), wherein, Z_(C) is said effective characteristic acoustic impedance of the acoustic impedance converter, ρ_(m) is the density of the metal layer, f_(p) is the center resonant frequency, V_(p) is the velocity of sound in the polymer layer, and ρ_(p) is the density of the polymer layer.
 17. The ultrasonic transducer of claim 15, further comprising a backing absorber layer coupled to said piezoelectric body, said backing absorber layer having a given characteristic acoustic impedance.
 18. A method for forming an ultrasonic transducer comprising the steps of: providing a piezoelectric body having a characteristic acoustic impedance; providing a backing absorber coupled to said piezoelectric body, said backing absorber comprising a low characteristic acoustic impedance material having a characteristic acoustic impedance lower than said characteristic acoustic impedance of said piezoelectric body; providing a polymer layer having a thickness t_(p1) coupled to the piezoelectric body; providing a metal layer for transmitting ultrasonic energy between a propagation medium having a characteristic acoustic impedance and said piezoelectric body through said polymer layer and having a thickness t_(m1) coupled to said polymer layer, wherein said characteristic acoustic impedance of said piezoelectric body is higher than said characteristic acoustic impedance of said propagation medium, and wherein said polymer layer and said metal layer define an impedance converter having a predetermined effective characteristic acoustic impedance intermediate that of said piezoelectric body and propagation medium characteristic acoustic impedances.
 19. The method of claim 18, wherein said thicknesses t_(p1) and t_(m1) are selected based on the densities of said metal layer and said polymer layer, the predetermined effective characteristic acoustic impedance, a center resonant frequency of the ultrasonic transducer and the velocity of the sound in said polymer layer.
 20. The method of claim 18, further comprising the step of providing a quarter wavelength matching layer for being in contact with the propagation medium, said quarter wavelength matching layer in contact with said metal layer.
 21. The method of claim 20, wherein said quarter wavelength matching layer comprises a polymer.
 22. The ultrasonic transducer of claim 7, wherein said front acoustic impedance converter has a resonant frequency, and said back acoustic impedance converter has a resonant frequency, wherein said resonant frequency of the front acoustic impedance converter is lower than the predetermined center resonant frequency of the ultrasonic transducer, and wherein said resonant frequency of the back acoustic impedance converter is higher than the predetermined center resonant frequency of the ultrasonic transducer.
 23. The ultrasonic transducer of claim 2, further comprising: a backing absorber coupled to said piezoelectric element by means of a back polymer layer substrate having a first planar surface on which is disposed an inner metal layer adapted as a signal trace for feeding current to the piezoelectric layer, and a second planar surface opposite the first planar surface and on which is disposed an outer metal shielding layer that is coupled to said backing layer, wherein said back polymer layer substrate and said outer metal shielding layer define a back impedance converter interposed between said backing absorber and said piezoelectric element. 